Quantum mechanically complete measurements in electron impact excitation of helium

A complete quantum description of the 3(1)D state of helium, that is the am plitudes and relative phases for the m = 0, +/-2 magnetic sublevels are rep orted. All state multipoles [T(2)(kq)] (k = 0-4, k greater than or equal to q greater than or equal to -k) that describe the population and the anisot ropy of the excited He(31D) state are presented. The data were determined f or 60eV incident electrons and 40 degrees electron scattering angle. The experimental procedure was to measure the triple-coincidence, polarisat ion correlations of the sequential cascading photons (667.8 nm 3(1)D --> 2( 1)P and 58.4 nm 2(1)P --> 1(1)S transitions) and the scattered n = 3 energy loss electrons. Simultaneous measurements of the three two-particle coinci dence pairs determined accurate magnitudes of the scattering amplitudes and phases as well as enabling consistency checks with earlier data. The tripl e coincidence data determined the sign of the relative m = +/-2 phases. The results are presented in a simplifying diagrammatic form. As yet there are no other complete experimental determinations of the 3(1)D state. The measurements are in agreement, within the experimental uncertainty, wit h the Convergent Close Coupling (CCC) calculations. Previous data for helium at 40eV incident elections are discussed in the co ntext of the present triple-coincidence results. The desirability of a rece ntly introduced parameter notation for the helium 3(1,3)D states is questio ned with respect to physical insight.